1. Field of the Invention
The present invention relates generally to fuel gas burners for low NOx production during combustion, and more particularly but not by way of limitation, to a burner assembly which has a burner plate assembly that reduces NOx combustibles.
2. Discussion
Oxides of nitrogen, collectively known as NOx, are major air-borne pollutants because of the harmful effects from acid rain, smog, vegetation destruction, ozone pollution, and health related issues. Traditionally, combustion, automobiles, and other fuel burning equipment are considered the primary sources of NOx emission. Boilers and steam generators have long been used for both industrial (food processing, heat treatment, etc.) and commercial (such as space heating) applications. As such equipment burns fuel with air to produce heat, significant amounts of pollutants, including NOx, are also produced.
The United States Environmental Protection Agency (EPA), established some 20 years ago by Congressional order, is the national regulatory authority on air quality. In November of 1990, on EPA's recommendations, Congress passed the Clean Air Act Amendments (CAAA). The processes producing pollutants were identified and timetables were established, setting limits on states to comply with air quality standards. The impact of CAAA has been felt by the boiler and burning manufacturing industry, as the owners and manufacturers of combustion equipment are potential regulatory targets for mandate control of NOx emissions limitations. Site permitting, regulatory compliance, and possible emission fees are viewed as potential consequences. In anticipation, the combustion industry is focusing its attention to curb NOx emission.
Several methods of NOx reduction have been devised, including both external and internal flue gas recirculation (FGR); steam injection (SI); fuel and air staging; premixing; controlled excess air firing; ceramic fiber; selective catalytic and non-catalytic reduction (SCR and SNCR); low NOx oil (for oil burning); and methods to introduce instabilities in the flow field, including the use of elliptical geometries, resonant generators, etc. These methods vary in cost as well as effectiveness in the reduction of NOx. From the perspective of a boiler/burner owner, the least expensive yet reliable method which enables the owner to reduce NOx to meet the prevailing air quality regulations is definitely the best for that application. The design improvements in NOx producing burners without expensive add-ons are certainly very appealing.
The formation mechanism of NOx is well described in combustion literature. It is now established that flames possess at least three possible routes of NOx formation: 1) Thermal NOx (Zeldovich's mechanism); 2) prompt NOx; and 3) fuel NOx.
In gaseous combustion, NOx formation from fuel NOx is insignificant, so only the first two mechanisms are of consequence. Thermal NOx is highly temperature dependent and often is the dominant mechanistic route. At temperatures below 1700 K.apprxeq.2600.degree. F. thermal NOx is insignificant, but it increases sharply above this temperature. Thermal NOx has a slow reaction rate constant, meaning it forms and dominates in the later portion of flames. Thermal NOx production is also sensitive to reactants (fuel and oxidizer) stoichiometry. Its peak occurs on the slightly fuel lean side of stoichiometry (.phi.&lt;1) and its concentration decays at off-stoichiometric conditions as either .phi. increases (fuel rich condition) or decreases (fuel lean condition). On the other hand, prompt NOx formation is a fast reaction and completes in the very early regions of flame, where fuel rich conditions exist and unbumt hydrocarbons, including the CN radicals, are in abundance.
Thus, from the above, it is clear that NOx formation (both thermal and prompt) in gaseous combustion can be reduced by: 1) Lowering flame temperature; 2) carefully setting the stoichiometry of fuel oxidizer (usually air) mixture; and 3) modifying the structure of flame in its early region so as to limit the formation of CN radicals.
The burner assembly of the present invention has demonstrated the ability to operate at sub 20 ppm levels of NOx without supplemental external reduction means such as flue gas recirculation, steam injection, or any other known means. The burner assembly incorporates a burner plate assembly that can be retro-fitted on conventional gas burners for low NOx operations, and is especially adapted for burners commonly employed in steam generators, commercial water heaters, absorption type water coolers/heaters and absorption type refrigerators.